This invention relates to light sensing devices and more specifically to light-pulse or light-change detection systems such as lightning detection systems using light-sensitive silicon devices.
In 1752, Benjamin Franklin arranged for corona current to ring bells fitted to an air gap in a long lightning conductor thereby making the first lightning detector of historical record. Since then, lightning detectors have improved and are now used for a variety of purposes.
Today, accurate and reliable lightning detection is needed more than ever. Every year, hundreds of people are injured or killed by lightning. Worldwide, annual property damage due to lightning now amounts to many billions of dollars.
Accurate and reliable lightning detection can prevent many of these injuries and damages. Lightning detectors can provide early warning of lightning threats to people and signal them to seek safe shelter. Lightning detectors can provide signals for the manual or automatic protection of valuable equipment from the direct and indirect effects of lightning. Operators of computer facilities can be alerted to use backup power supplies before power interruptions and transients due to thunderstorms occur.
Lightning detectors are finding many new uses. They have recently been used to help predict "microbursts" associated with thunderstorms. Microbursts are the suspected cause of several deadly aircraft accidents.
There are several well-known methods used to detect lightning. Some lightning detectors (traditional "flash counters" and sferics detectors) are based on sensing the radiated fast or slow electric field changes that are typical of lightning. These detectors use frequency-selective filters to bias their response in favor of lightning signals. These detectors are particularly limited by poor noise discrimination and consequent false lightning indications.
Some lightning detectors are based on vibrating-reed or rotating-vane electrostatic flux meters (field mills). These detectors sense the steady level and slow variations of electric fields as clouds nearby become electrified, produce lightning and dissipate or recede. Field mills are expensive, limited in detection range, mechanically complex, and can at times indicate "thunderstorm" during a dust storm or other non-threatening situation.
Other lightning detectors are based on point-discharge (corona-current) sensors in which field-emissions from a sharp elevated conductor are amplified to give an indication of the sign and magnitude of the vertical electric field and electric field changes. Corona-current sensors are less complex than field mills, but can give false hazard indications (or fail to give hazard indications) when space charge and the motion of space charge near the ground interferes with measurement of the charge aloft.
Gated, wideband, magnetic direction finders as described in U.S. Pat. Nos. 4,198,599, 4,115,732, and 4,806,851 issued to Krider et al., are in wide use throughout the world. These detectors are biased to trigger on the signals radiated by ground discharges. They measure bearing angles and other parameters of ground discharges. Single-station versions of these sensors provide estimates of the range to ground discharges. Single-station direction finders are relatively expensive and must be carefully located to prevent false alarms and minimize errors in direction measurement.
Low-cost, hand-held electrical, optical, and electro-optical lightning detectors have come into use in recent years. These detectors require an alert, educated operator to make measurements and assess hazards. Sensors that detect only optical radiation or only electric field changes may give false alarms with natural and man-made variations in light or when used near noise sources such as electronic and electrical equipment. These detectors often require relatively complex signal processing and power supply circuits. Some of these instruments are highly directional and must be pointed.
Lightning coincidence detectors are well known in the art. Lightning coincidence detectors are discussed in: Journal of Geophysical Research, Vol. 65, No. 7, pg. 1927-1931, July 1960, "Electric-Field Changes and the Design of Lightning Flash Counters" by M. Brook and N. Kitagawa; Conference Proceedings of the 1991 International Aerospace and Ground Conference on Lightning and Static Electricity, pg. 100-1 to 100-3, "Portable Combined Optical and Electric Field Change Intracloud Lightning Detector," by R. Markson; and, in a paper presented at the 21st International Conference on Lightning Protection, 1992, Berlin, titled "An Electro-Optical, Lightning Detection, Classification and Ranging Sensor for Automatic Lightning Protection and Human Warning", by L. G. Byerley, A. Pifer, and K. Cummins [all incorporated hereto by reference].
Commercially, lightning coincidence detectors are available from Lightning Location and Protection, Inc. (Tucson, Arizona), and Airborne Research Associates (Cambridge, Massachusetts).
Some complex solutions to lightning detection are U.S. Pat. No. 4,095,221, titled "Electrical Storm Forecast System", issued Jun. 13, 1978 to Slocum, Jr; U.S. Pat. No. 3,715,660, titled "Determining Distance To Lightning Strokes From A Single Station", issued Feb. 6, 1973 to Ruhnke; and, U.S. Pat. No. 4,422,037, titled "Storm Warning Method and Apparatus", issued Dec. 20, 1983 to Coleman which teach much more complex solutions to the problem of lightning detection.
Clearly, each present lightning detection technique is lacking in one or more important capabilities. A need exists for a new type of short-range lightning detection device that is simple, reliable, accurate, free of false alarms, immune to noise, easily deployed and inexpensive.